The present invention relates to a method for producing a shaped sintered porous body exhibiting a large surface area to volume ratio, a three-dimensional complex shape and an open three-dimensional porosity with a large surface area to volume ratio. The invention further relates to bodies produced by the method and the use thereof, such bodies for use as reactor elements, e.g. reactors for the chemical and process industry.
By way of example, commercial distillation is normally practiced as a multistage, counter current gas and liquid operation in a tower containing devices such as packing to facilitate the gas-liquid contacting that is necessary for both mass and heat transfer. The sintered porous body of the present invention is, in one application, for use as such a packing device. The term mass transfer relates to the contact efficiency of one medium to another and specifically refers to material moving from one phase to another. In a distillation process for example no reaction may be ocurring, however, improved contacting of one fluid, e.g., a liquid with another fluid, a gas or liquid, or a gas with a gas or a liquid with liquid is desired in the distillation process.
Since multiple equilibrium stages exist in a distillation tower, the compositions of the vapor and the liquid change throughout the tower length. The desired products can be removed as either liquid or vapor at an optimum location in the tower. The more efficient he mass transfer, the shorter the tower or more energy efficient the tower to achieve the same number of equilibrium stages. Mass transfer devices of the prior art typically are separated trays which allow vapor to pass upwards through a small height of liquid or continuous packings which contain surfaces for gas-liquid contacting. The advantage of structured packings in distillation processses are high efficiency coupled with low vapor pressure drop. Low pressure drops are desired because of the increased cost to force gases upwardly in the tower to overcome high pressure differentials, if present, and also because high pressure differentials tend to result in column xe2x80x9cflooding,xe2x80x9d where the liquid can no longer pass down the column.
Efficiency in a catalytic converter depends upon efficient contact of one fluid with another (gases or liquids in various combinations) or with a catalyst (a solid) and so on. Also, improved contact, i.e., mass transfer, is desired between fluids and/or fluids and solids since reaction rate depends upon th efficiency of th mass transfer with a solid catalyust. Thus improved contacting or mixing is desired to provide enhanced mass transfer or reaction rate in accordance with a given implementation. Sorption is thus desired in these processes whether adsorption or absorption.
Examples of catalytic distribution structures are disclosed in U.S. Pat. No. 4,731,229 to Sperandio, U.S. Pat. No. 5,523,062 to Hearn, U.S. Pat. No. 5,189,001 to Johnson, and U.S. Pat. No. 5,431,890 to Crossland et al.
Improved prior art packing structures have been developed comprising composite substrate structures, sometimes referred to as micromesh, which are porous products comprising a fibrous network of material. US Pat. Nos. 5,304,330; 5,080,962; 5,102,745 and 5,096,663, incorporated by reference herein, disclose the production of porous composite substrates comprising fibrous networks of material. A substrate mixture is comprised of typically metallic fibers for forming the porous composite and a structure forming agent which functions as a binder, which are dispersed in an appropriate liquid. After preforming, the liquid is removed and the composite heated to effect sintering of the fibers at junction points to produce a porous substrate composite comprised of a three-dimensional network of fibers. The structure forming agent is removed during or after sintering.
However, the porous material substrate in a packing structure of the type described above does not normally provide for fluid communication through the pores for the gases and liquids in the distillation process to provide for the needed desired contact while maintaining the desired low pressure drop. This is attributed to possibly capillary action due to the substrate material relatively small pore size. Such material may be for example 100 micron thick sheets (generally about 0.5-0.075 mm thick in one or more layers according to the desired strength) having the stiffness of conventional cardboard material, and sometimes referred to as a xe2x80x9cpaper,xe2x80x9d although comprising metal fibers and stronger than paper of cellulose fibers. Such material has a high surface to void volume, comprising approximately 90-98% voids.
One common design criteria for reactor elements is that they preferably offer a high surface area to volume ratio which in most cases is combined with a low pressure drop in the reactor. This is to optimize mass transfer. To increase this ratio, reactor elements are in the form of assemblies made from sheet material which have been press-formed into corrugations and so on and joined to form a system of larger internal passage-ways or channels. The sheets may be solid, with or without openings, i.e., perforations, or have an open mesh-like structure. Mesh-like metallic sheets with a woven cloth-like structure are used to provide catalytic surfaces. Reactor elements with a catalytic function typically comprise a catalytic material, which may be included as wire in the mesh but are typically applied as a coating or deposition on the mesh.
Before assembling the reactor elements in a reactor, the sheet material is often shaped by pressing to simple shapes like corrugated sheet and the like or punched to provide holes or openings. This shaping is a costly additional step and requires extreme care in handling delicate porous paper-like porous sheet material. The shaped sheets are subsequently assembled either in a framework or by joining using suitable methods such as soldering and/or spot-welding.
In accordance with the production of fibrous mesh in the latter above-noted US patents, a mixture is provided comprised of fibers for forming the porous composite body and a structure forming agent, in particular cellulose fiber, which function as a binder, which are dispersed in an appropriate liquid. After performing into a desired form, liquid is removed and the composite body is heated to a temperature to effect sintering of the fibers at junction points to produce a porous composite body comprised of a three-dimensional network of fibers. The structure forming agent is removed before or during the sintering process or may be removed after the sintering process.
Japanese Patent Publication JP-A-95/97 602 discloses sheets of the non-woven type comprising metallic fibers and formed with methods similar to conventional paper-making methods and subsequently subjected to heat for removal of paper fiber and formation of thermal bonds between the metallic fibers. The sheets are subsequently shaped or assembled to bodies suitable for use as reactor elements. These sheets are preferably coated with catalytic material.
Also this type of sheet material is shaped before assembly to reactors, using the same shaping, assembling and joining methods discussed above. These shaping processes typically are mechanical, employing dies and the like which are cumbersome and may be complex and costly to implement. The shaping of fibrous non-woven sheets is difficult as there is a need to avoid damaging the sheet material during formation. Such shaping at a minimum thus requires the sheet material to be formed and then later processed to produce the three-dimensional shape, such as corrugations and the like needed for the reactor processes.
However, with known bodies and methods for producing such bodies there are still restrictions in the freedom in design of bodies and in particular for use in reactor environments, and there is still a long felt need for an increase in surface area to volume ratio for many applications. There is also a need for new manufacturing methods for such reactor elements offering simple forming and shaping of the reactor elements and thereby offering cost reductions. This need is in particular long felt for reactors in the form of assemblies comprising a system of internal passage-ways including channels and walls. The need is for walls that exhibit an open porosity and, when appropriate, larger holes or perforations, which provide openings for fluid passage between the system of internal channels. Such shaping and assembling of the sheet material to assemblies with desired external and internal shapes in prior art systems is elaborate and costly.
In particular new methods are needed for manufacturing bodies with an open internal structure exhibiting internal passage-ways with a multiplicity of channels defined by internal walls having an open porosity and preferably holes or perforations providing openings between the channels. Also, known techniques offer no or limited possibilities to optimize both the size and shape of the pore structure in the internal walls of bodies and/or the larger passage-ways, i.e. the channels in the reactor elements to best meet the individual requirements of an installation. The need is also for introduction of catalytic material into the internal passage-ways by more efficient methods.
It is an object according to the present invention to provide methods for manufacturing a shaped sintered porous body for use, for example, as reactor elements.
A method for producing a shaped three-dimensional sintered porous body according to the present invention comprises the steps of:
dispersing and suspending a fiber mixture comprising inorganic fibers and a liquid containing an organic binder to form a fibrous slurry;
forming a wet fibrous body comprising the fiber mixture;
shaping the wet fibrous body to a three-dimensional configuration;
removing at least a portion of the liquid from the fibrous body;
drying the fibrous body;
heating in a first heating step the dried fibrous body to a first elevated temperature to remove the binder from the body;
subsequently heating the dried body after the first heating step to a second elevated temperature to fuse the inorganic fibers and, upon cooling, to form a sintered porous fibrous body with a three-dimensional configuration.
In one aspect, the fiber mixture is dispersed and suspended in water to form a water-based slurry and that the slurry is de-watered during or in connection with the forming and/or shaping of the fibrous body.
In a further aspect, the binder is selected from any one of the group consisting essentially of cellulose, organic resins including polyvinyl alcohol, polyurethanes, and styrenebutadiene latex, and thermosets, including epoxies, ureaformaldehyde resins, melamine-formaldehyde resins, and polyamide-polyamine epichlorohydrin resins
In a further aspect, the method includes forming the wet fibrous body into a sheet and then pressing the fibrous wet sheet into the three-dimensional configuration.
The forming and shaping preferably comprises a one-step molding operation.
The method in a further aspect may include the step of spraying the slurry onto a mold and simultaneously de-watering the sprayed mold to form a de-watered fibrous body. The forming the fibrous body in a further aspect comprises forming a mold comprising a molding surface with an open permeable structure and dipping the mold into the slurry, applying a suction to the mold such that the slurry is de-watered through the mold and forming the fibrous body on the mold.
The slurry in a further aspect is injected into a mold cavity exhibiting at least one molding surface with an open permeable structure, forming the fibrous body within the mold cavity and applying a suction to the mold cavity such that the slurry contained in the mold cavity is de-watered and the fibrous body is formed.
In a further aspect, the ratio of binder to inorganic fiber in the fiber mixture is controlled such that the shaped sintered body exhibits a predetermined porosity.
The sintered fibrous body in a still further aspect is coated with a catalyst in a subsequent step following the sintering.
The inorganic fibers are preferably selected from the group consisting essentially of metallic fibers, ceramic fibers, glass fiber, carbon fiber or any combination thereof.
The catalytic material in a further aspect is dispersed and suspended in the slurry.
The sintering heating step preferably includes heating the shaped body at a temperature below the melting point of the inorganic fibers but sufficiently high to achieve a partial fusion and a sintering of the contact surfaces between the fibers.
In a further aspect, a shaped, sintered, porous three-dimensional body is that produced according to the method.
The body in a further aspect exhibits a plurality of shaped porous sections having a broad surface and a thickness that is substantially thin relative to the broad surface.
The body shaped thin-walled porous sections exhibit a porosity preferably exceeding about 50 percent by volume and more preferably exceeding about 90 percent by volume.
The body may be coated with catalytic material.
The inorganic fibers are preferably selected from the group consisting essentially of one or more of metallic fibers, ceramic fibers, carbon fiber, glass fiber and/or combinations thereof.
The body is preferably arranged in one aspect as a reactor element to provide gas/condensed phase mass transfer areas in a system comprising any one of the group of processes consisting essentially of processing or treatment of gases and/or vapors through distillation, absorption and catalytic reactions requiring gas/condensed phase mass transfer.
The body in a further aspect is arranged as a reactor element to provide fluid-solid mass transfer, or to promote fluid-fluid mass transfer, or combinations thereof in a system comprising any one of the group of processes consisting essentially of processing fluids through ab- and ad-sorption and catalytic reactions.
In a still further aspect, the method for producing a shaped three-dimensional sintered porous body comprises the steps of:
forming a fibrous body of a wet fiber mixture comprising inorganic fibers and a liquid containing an organic binder in a three-dimensional configuration;
subsequently drying the body; and then heating the body to remove the organic binder and to sinter the inorganic fibers to form a three-dimensional shaped body comprising a porous network of interconnected inorganic fibers.
The method preferably includes in a further aspect shaping the body while wet during the forming the fibrous wet body in a single step into a three-dimensional shaped sheet member.
The method in a further aspect includes forming apertures and fins on the body during the forming.
The method preferably including drying the body after the forming, the heating including a first heating step at a first temperature to remove the inorganic binder and then heating the body at a second elevated temperature to partly fuse the inorganic fibers to form a sintered porous fibrous body with the three-dimensional shape.
The method may include storing the dried body, soaking the body after storing to form a wet body and then shaping the wet body prior to the heating.
The method according to the present invention offers capabilities for a cost-effective processing, but more important, a high degree in flexibility in design and dimensioning of the passage-ways, including the larger channels and the open porosity and in the choice of both functional and structural materials to be included in the shaped, sintered, porous, fibrous bodies. Thus, as an extra advantage, the present invention provides an increase in the performance limitations of the bodies, e.g. temperature range, compositions of the gases or vapors to be processed. Thereby the bodies produced by the methods of the present invention are useful for new applications.
The bodies when intended for use in a reactor, such as a catalytic reactor, can be employed for a wide variety of chemical reactions. As representative examples of such chemical reactions, there may be mentioned hydrogenation reactions, oxidations, dehydrogenation reactions, alkylation reactions, hydrotreating, condensation reactions, hydrocracking, etherification reactions, isomerization reactions, selective catalytic reductions, and catalytic removal of volatile organic compounds.